It is known to use non-conductive bearings or sleeves in conveyor rollers and other rollers in various process equipment to provide non-corrosive assemblies and to reduce costs for such equipment. A non-conductive bearing is known to be composed of nylon, polyurethane or other synthetics for durability and anti-corrosion properties. Such non-metallic bearings do not require grease or other lubrication and are often preferred for machinery used in food processing equipment, medical supplies production facilities and other installations that require protection from contamination.
A disadvantage from the use of such non-conductive bearings on rollers and shafts is that static charges are generated and stored in the roller or shaft, particularly under high speed operating conditions. The static charges result from the separation of two sliding surfaces that are not conductive and are isolated. Although it is common to form the shaft of metal, a metal shaft will store static charges if the shaft is isolated by non-conductive bearings. When the charge level of the shaft reaches a critical limit, a spark can occur, arcing to conductive components that are nearby. It is known in such installations that the static charge may reach twenty thousand volts, and can destroy critical electronic components near the conveyer roller or shaft, and may shock personnel in close proximity thereto.
Another common occurrence of shaft-induced current is experienced with three-phase motors utilizing variable speed control devices. Static charges can accumulate to a level that overcomes air insulation, approximately 3,000 volts/mm between the shaft and bearing races. An instantaneous burst of electrical energy can occur, draining the high level current to ground. Arcing creates potential danger to persons nearby and can cause damage to nearby components, including sputtering of metal from the bearing races at the point of current flow, thus damaging the bearing.
It is known to provide conductive grounding brushes in contact with a roller or shaft that becomes charged during operation. The brush is electrically grounded, providing a path to ground for a static charge that would other wise build in the roller or shaft. A disadvantage of the use of such discharge brushes is the difficulty of ensuring contact of the fixed position brush with the moving surface of a rotating roller or shaft. The sliding contact between the brush and the rotating shaft or roller also causes significant and relatively rapid wear of the brush, which can generate dust or liberate fibers from the brush. Contamination from such dust or fibers can be detrimental to the process in which the roller or shaft operates.
In electric motors, it is known to use spring-loaded copper brushes to provide a substantially continuous flow of current to ground. While copper brushes work well to control the build-up of static charges, the copper brushes wear out quite rapidly and require frequent service and/or replacement. Copper brushes can allow a burst of energy on power-up because of metal-to-metal contact design.
What is needed in the art is a static neutralizing device for conveyor rolls, motor shafts and the like, that can be used easily to dissipate static charges effectively, and which eliminates intimate contact between the device and the roller or shaft to minimize wear and the need for service or replacement.